Cutting inserts with several cutting teeth simultaneously engaged in an incrementally cutting thread profile of increasing depth are used particularly in the manufacture of internal and external threads on pipes used in the oil and natural gas industries. These types of threads must be manufactured to extremely close tolerances. The close tolerances demand a very high degree of dimensional accuracy in the cutting inserts. This is particularly true for the last or finish cutting tooth, which makes the finishing cut on the thread.
The production of these cutting inserts out of durable material, usually a hard metal, involves a powder metallurgical process. Traditionally, pressing and sintering have been used to form a blank of the cutting insert without shaping the individual cutting teeth. After the blank is formed, the cutting inserts are surface ground on the back face and the front face serving as chip removing plane to produce the thickness of the cutting insert within the required tolerances. Then, the cutting teeth are created by grinding several blanks in a stack with a profile grinding wheel. Thus, the final dimensions for profile depth and tooth width of the individual cutting teeth are achieved in a single grinding operation.
Using this process the flanks of the side cutting edges of the individual cutting teeth can only be ground parallel to one another. As a result, the necessary cutting relief angles of these cutting edges are achievable in a limited quality only.
These relief angles are achieved by the process of tilting the cutting inserts before using the profile grinding wheel, in order to achieve the necessary relief angles of the flanks of the end cutting edges of the individual cutting teeth. Then when the cutting inserts are held by the toolholder at a rake angle of less than 0.degree., this also results in relief angles of the flanks of the side cutting edges of the individual cutting teeth because the side cutting edges each form angles of over 90.degree. with the end cutting edge.
Particularly in the case of cutting inserts on which these angles exceed 90.degree. by only a small amount, the relief angle of the side cutting edges is also very small and often far from the optimal value of approximately 6.degree. which is necessary for cutting. This results in rapid wear of these side cutting edges.
Furthermore, these relief angles cannot be increased freely by tilting the cutting inserts to a greater degree for the profile grinding operation, since then the front rake, and with it the mechanical stability of the end cutting edges, would be too small.